How to render parts of the camera view and merge afterwards?
Using python is probably the best way to split up the image into exact tiles, as mentioned by Campbell Barton:


  If you do some operation that requires precision (splitting the render into exact tiles for eg), you would be best to use Python for this anyway


At the moment you can try the Render Border Add-on. Although it's built to (re-)render an exact area of the image in order to save render time. For a single image you can animate 'Center X' and 'Center Y' properties to get the desired tiles:



To get 4 Tiles for instance:


Hit 'Reset the Render Border' to get the full frame
Divide 'R' and 'T' by 2 to get the first tile
Multiply 'Center X' by 3 to get the tile in the lower right corner
Then multiply 'Center Y' by 3 to get the upper right tile
Finally divide 'Center X' by 3 to get the tile in the upper left corner


Note: Unfortunately you cannot render this as animation, since Blender checks the size of the render border only once per render execution. So you'll need to render single frames and stitch it afterwards (by hand or using image tools such as ImageMagick):


Result



Also I'd highly recommend Brecht's instructions on how to render large image resolutions: https://blender.stackexchange.com/a/16261/3710